The way I see it is, you have two choices - (a) the brightest possible colour, which will almost certainly not be accurate, or (b) the most accurate possible colour, which will almost certainly not be the brightest the press can produce. In most cases, we pursue choice (b) - that's the nature of colour management. However, I have certainly chosen to pursue choice (a) for various jobs in the past.

If you pursue choice (b), then you need to know that the ICC profile you're using is accurate - hence my original question. Use your soft-proofing and gamut warning, and massage your o-o-g colours into the destination gamut. By the way, I'm not sure about your global 40% desaturation, I'd prefer to see you desaturating particular colours as needed. In Photoshop's Hue/Sat dialog, use the Sat slider to desaturate each colour enough to get rid of those "!" marks in your Info palette, and where necessary, nudge the Hue slider to keep the colour as true as possible to the original. Just like TommyO said, it's better to do this yourself, and keep exquisite control over it, than let an automatic process control it for you.

If you pursue choice (a) - that is, saturation at all costs, regardless of colour accuracy - then I would still do the careful desaturation as described above, then convert, then do whatever it takes to get your colours to their max (eg reds 100M100Y, blues 100C100M etc). This might mean Curves, or Levels, or Selective Color, or Hue/Sat (although I've found Hue/Sat to be awkward in these cases). The point is, once you start trying for those extreme CMYK colours, you'll be departing from the colours in your original file.